It's wrong for a bride to dictate what gift she will receive for a wedding (or bridal shower). It's also rude to be a tight@r$e guest. On average I would consider $20 to be pretty cheap... Unless that person was in exceptional circumstances (on a pretty low income). If a guest at my wedding didn't have much $$ I would rather they enjoy themselves and not give me anything.
I say now I'd like to judge the genuine tight@r$es but in reality, years later I can't remember who have what at my wedding. I remember who was good company and who wasn't.